1. Field of the Invention
This invention relates to an optical device which employs an optical part wherein a spherical lens is force fitted in a cylindrical lens holder, and more particularly to an optical switch which includes an optical part of the type mentioned.
2. Description of the Related Art
In the field of optical communications or optical transmissions, a spherical lens is widely applied in order to convert light emitted from a light emitting device or an emergent end of an optical fiber into a parallel beam of light or reversely to focus a parallel beam of light to a light detecting device or an incident end of an optical fiber. In such applications of a spherical lens, a structure is demanded which can fixedly hold a spherical lens firmly therein because the relative positional relationship between a spherical lens and either a light emitting device, a light detecting device, an optical fiber has a direct influence on the optical coupling efficiency of an optical device.
Conventionally, in fixing a spherical lens which has a spherical outer profile and is not necessarily easy to handle, the spherical lens is, for example, force fitted into a cylindrical lens holder so as to hold the spherical lens in the lens holder, and the spherical lens is fixed together with the cylindrical lens holder to a substrate by laser welding or the like. To better hold the spherical lens in the lens holder, the diameter of the spherical lens is set a little greater than the diameter of a receiving hole formed in the lens holder. Accordingly, the spherical lens can be held at a predetermined position in the lens holder by pushing the spherical lens into the receiving hole with a suitable force. Such holding structure of the spherical lens by force fitting assures firm holding of the spherical lens making good use of plastic deformation and/or elastic deformation of the lens holder. However, the holding structure has the following drawbacks.
In particular, where the material of the lens holder is a soft metal material such as invar, covar or brass, the lens holder undergoes plastic deformation by force fitting of the spherical lens. The lens holder swells at the force fit portion thereof so that the outer profile of the lens holder is nonuniform. Accordingly, it is a problem that, when the lens holder is closely contacted with and fixed to a flat substrate, then the center axis of the lens holder will not provide a parallel relationship to the flat surface of the substrate. On the other hand, where a hard metal material such as stainless steel is used as a material of the lens holder in order to omit corrosion preventing processing which is required for such a soft metal material as described above or in order to permit fixation of the lens holder to the substrate by welding, there is another problem, in addition to the problem described above, that the spherical lens is likely to be broken. In order to eliminate this, the accuracy in dimension of the diameter of the spherical lens and the diameter of the receiving hole of the lens holder must necessarily be raised specially, which makes a problem that the production cost is raised considerably.
An optical part wherein a spherical lens is force fitted into a cylindrical lens holder from an end of the lens holder and a ferrule to which an optical fiber is connected is fitted in and fixed to the other end of the lens holder is often used as an optical part which employs a spherical lens. Such optical part will be hereinafter referred to as a fiber collimator. Such a fiber collimator is frequently used, for example, in an optical switch for changing over a light path. Optical switches are widely used as a basic optical device, and optical devices are demanded which are high in reliability and suitable in miniaturization.
Conventionally, various types of optical switches for changing over an optical path have been proposed including a type wherein a movable prism is selectively inserted into a light path and another type wherein the coupling coefficient of a directional coupling device is changed depending upon an electro-optical effect. Among optical switches of the types mentioned, a conventional mechanical optical switch wherein a movable prism is selectively inserted into a light path has a drawback that the entire switch mechanism inevitably has a large overall size and another drawback that the problem of incomplete switching cannot be avoided which may be caused by particles produced by abrasion of a sliding portion of the switch or by dust or the like admitted into the switch. Particularly the problem of incomplete switching caused by dust is significant where the switching interval is very long.